Communication devices such as User Equipments (UE) are also known as e.g. mobile terminals, wireless terminals and/or mobile stations. User equipments are enabled to communicate wirelessly in a wireless communications network, sometimes also referred to as a wireless communication system, a cellular communications network, a cellular radio system or a cellular network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
User equipments may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The user equipments in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another user equipment or a server.
The wireless communications network covers a geographical area which is divided into cell areas, wherein each cell area is served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB” or “B node” depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the user equipment. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the user equipment to the base station.
In some RANs, such as in the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), several base stations may be connected, e.g. by landlines or microwave, to each other.
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE is controlled by the radio base station.
The 3GPP is currently working on further standardization of the LTE concept. The architecture of the LTE system is shown in FIG. 1a, including radio access nodes (eNBs) and interfaces to evolved packet core nodes, such as Mobility Management Entity (MME) and Serving Gateway (S-GW). The interface between eNBs is referred to as X2, and the interface between eNB and MME/S-GW is denoted S1. The application layer signalling, on the S1 interface, S1-AP, is standardized in 3GPP TS 36.413, and on the X2 interface, X2-AP, in 3GPP TS 36.423.
Carrier Aggregation
Aggregation of carriers is supported for LTE since 3GPP Release-10 (LTE Rel-10) with individual bandwidths up to 20 MHz, which is the maximal LTE Rel-8/9 carrier bandwidth. Hence, an LTE Rel-10 operation wider than 20 MHz is possible and appear as a number of LTE carriers to an LTE Rel-10 user equipment. Carrier aggregation is an optional feature in LTE Rel-10 with enhancements to this feature added in 3GPP releases later than 10.
The LTE Rel-10 standard supports signalling for up to 5 aggregated carriers where each carrier is limited in the RF specifications to have one of six bandwidths, namely 6, 15, 25, 50, 75 or 100 Resource Blocks (RB), corresponding to 1.4, 3, 5, 10, 15 and 20 MHz respectively.
Carrier aggregation is called contiguous or non-contiguous intra-band if the carriers all belong to the same 3GPP operating frequency band.
Inter-band carrier aggregation is the case when there is at least one carrier in a different 3GPP operating frequency band.
The number of aggregated carriers, also called Component Carriers (CC), as well as the bandwidth of the individual CC, may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same, whereas an asymmetric configuration refers to the case when the numbers of CCs are different in downlink and uplink. However, an asymmetric configuration where the number of uplink CCs is higher than the number of downlink CCs is not allowed in LTE Rel-10.
It is important to note that the number of CCs configured in the network may be different from the number of CCs seen by a user equipment. A user equipment may for example support more downlink CCs than uplink CCs, even though the network offers the same number of uplink and downlink CCs. From a network perspective, in LTE Rel-10 all cells should be Rel-8/9 compatible, for example they must comprise one uplink and one downlink CC. In LTE Rel-11 and later, new carrier types which are not Rel-8/9 compatible are supported, for example that a cell of a certain frequency band may comprise a downlink CC only from a network perspective.
During initial access, an LTE Rel-10 user equipment behaves similar to an LTE Rel-8/9 user equipment. The serving cell, in which the UE ends up at initial access, is referred to as the UE's Primary Cell (PCell).
Upon successful connection to the network, a user equipment may—depending on its own capabilities and depending on the network—be configured with additional CCs in the UL and DL. These additional serving cells, which are configured for the UE, are referred to as Secondary Cells (SCell).
Downlink-Only Cells for the Purpose of Carrier Aggregation
In LTE Frequency Division Duplex (FDD) systems, most frequency bands are defined with one uplink band and one downlink band, to facilitate two-way communication.
In the special case of carrier aggregation, a UE is connected to a cell using a set of frequencies but may also simultaneously use a second cell using another set of frequencies. The second cell may by the UE either be used for both UL and DL transmission or only be used for DL transmission. Typically, the cell used as a secondary cell, has both an UL and a DL carrier from an eNB point of view, even if the UE is configured to only utilize e.g. the DL carrier of this cell. Then the eNB will still have an UL frequency configured for this cell and will listen to it, even if not all UEs are configured to have any UL transmission on this frequency.
There is however special cases where a cell from an eNB point of view may be deployed with only a DL. This means that the eNB is typically not listening on the UL frequency associated with the DL frequency of this cell or is ignoring it. Alternatively, the DL frequency has no associated UL frequency. In the 3GPP standards there is currently only one frequency band defined as DL-only, i.e. band 29 as defined in 3GPP TS 36.101, meaning that it has no UL E-UTRA Absolute Radio Frequency Channel Number (EARFCN) defined, only a DL EARFCN. Other bands may be defined in the 3GPP standards with both UL EARFCN and DL ERAFCN, but it may for the operator anyway be desirable to use these bands for a DL-only cell. Cells deployed as DL-only cells from a network perspective may only be utilized for the purpose of secondary cells providing additional resources for DL transmission. In 3GPP terminology, what is referred to as a DL-only cell in this document, is sometimes referred to as a Supplementary Downlink (SDL) cell.
A UE ordered to do measurements for finding neighbour cells, cannot see any difference between cells configured for both UL and DL and cells configured for DL only. It will report all neighbour cells that fulfil the measurement criteria as possible candidates for mobility actions.
When the Automatic Neighbour Relations (ANR) function in an eNB finds the DL-only cell in a frequency band defined for both uplink and downlink, e.g. in a frequency band other than band 29, the DL-only cell is automatically configured as a neighbour cell. The eNB assumes that the DL-only cell supports both UL and DL transmission and thus is available for handover. However, all attempts to make handover to this neighbour DL-only cell will fail. Further results are inconsistent network configuration, i.e. the DL-only cell will be described as configured for DL and UL in the neighbouring eNBs and as configured for DL-only in the serving eNB, connection drops and unnecessary load on network nodes.